Network Working Group A. Bryan, Ed.
Internet-Draft T. Tsujikawa
Intended status: Standards Track N. McNab
Expires: April 7, 2010 Metalinker Project
P. Poeml
Novell, Inc.
October 4, 2009
The Metalink Download Description Formatdraft-bryan-metalink-18
Status of this Memo
This Internet-Draft is submitted to IETF in full conformance with the
provisions of BCP 78 and BCP 79. This document may contain material
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This Internet-Draft will expire on April 7, 2010.
Copyright Notice
Copyright (c) 2009 IETF Trust and the persons identified as the
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Internet-Draft Metalink Download Description Format October 20091. Introduction
Metalink is an XML-based document format that describes a file or
list of files to be added to a download queue. Metalinks can list a
number of files, each with an extensible set of attached metadata.
For example, each file can have a description, checksum, and list of
URIs that it is available from.
Identical copies of a file are frequently accessible in multiple
locations on the Internet over a variety of protocols (FTP, HTTP, and
Peer-to-Peer). In some cases, Users are shown a list of these
multiple download locations (mirrors) and must manually select a
single one on the basis of geographical location, priority, or
bandwidth. This distributes the load across multiple servers. At
times, individual servers can be slow, outdated, or unreachable, but
this can not be determined until the download has been initiated.
This can lead to the user canceling the download and needing to
restart it. During downloads, errors in transmission can corrupt the
file. There are no easy ways to repair these files. For large
downloads this can be extremely troublesome. Any of the number of
problems that can occur during a download lead to frustration on the
part of users.
All the information about a download, including mirrors, checksums,
digital signatures, and more can be stored in a machine-readable
Metalink file. This Metalink file transfers the knowledge of the
download server (and mirror database) to the client. Clients can
fall back to alternate mirrors if the current one has an issue. With
this knowledge, the client is enabled to work its way to a successful
download even under adverse circumstances. All this is done
transparently to the user and the download is much more reliable and
efficient. In contrast, a traditional HTTP redirect to a mirror
conveys only extremely minimal information - one link to one server,
and there is no provision in the HTTP protocol to handle failures.
Other features that some clients provide include multi-source
downloads, where chunks of a file are downloaded from multiple
mirrors (and optionally, Peer-to-Peer) simultaneously, which
frequently results in a faster download. Metalinks also provide
structured information about downloads that can be indexed by search
engines.
[[ Discussion of this draft should take place on
discuss@apps.ietf.org or the Metalink discussion mailing list located
at metalink-discussion@googlegroups.com. To join the list, visit
http://groups.google.com/group/metalink-discussion . ]]
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which this specification uses internally.
1.3. Notational Conventions
This specification describes conformance of Metalink Documents.
Additionally, it places some requirements on Metalink Processors.
This specification uses the namespace prefix "metalink:" for the
Namespace URI identified in Section 1.2, above. Note that the choice
of namespace prefix is arbitrary and not semantically significant.
Metalink is specified using terms from the XML Infoset
[REC-xml-infoset]. However, this specification uses a shorthand for
two common terms: the phrase "Information Item" is omitted when
naming Element Information Items and Attribute Information Items.
Therefore, when this specification uses the term "element," it is
referring to an Element Information Item in Infoset terms. Likewise,
when it uses the term "attribute," it is referring to an Attribute
Information Item.
Some sections of this specification are illustrated with fragments of
a non-normative RELAX NG Compact schema [RELAX-NG]. However, the
text of this specification provides the definition of conformance. A
complete schema appears in Appendix B.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in BCP 14, [RFC2119], as
scoped to those conformance targets.
2. Metalink Documents
This specification describes Metalink Documents.
A Metalink Document describes a file or group of files, how to access
them, and metadata that identifies them. Its root is the metalink:
metalink element.
namespace metalink = "urn:ietf:params:xml:ns:metalink"
start = metalinkMetalink
Metalink Documents are specified in terms of the XML Information Set,
serialized as XML 1.0 [REC-xml] and identified with the "application/
metalink4+xml" media type.
Metalink Documents MUST be well-formed XML. This specification does
not define a DTD for Metalink Documents, and hence does not require
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them to be valid (in the sense used by XML).
Metalink allows the use of IRIs, encoded according to [RFC3987].
Every URI [RFC3986] is also an IRI, so a URI may be used wherever
below an IRI is named. There is one special consideration: when an
IRI that is not also a URI is given for dereferencing, it MUST be
mapped to a URI using the steps in Section 3.1 of [RFC3987].
Any element defined by this specification MAY have an xml:base
attribute [REC-xmlbase]. When xml:base is used in an Metalink
Document, it serves the function described in Section 5.1.1 of
[RFC3986], establishing the base URI (or IRI) for resolving any
relative references found within the effective scope of the xml:base
attribute.
Any element defined by this specification MAY have an xml:lang
attribute, whose content indicates the natural language for the
element and its descendents. The language context is only
significant for elements and attributes declared to be "Language-
Sensitive" by this specification. Requirements regarding the content
and interpretation of xml:lang are specified in XML 1.0 [REC-xml],
Section 2.12.
metalinkCommonAttributes =
attribute xml:base { metalinkUri }?,
attribute xml:lang { metalinkLanguageTag }?,
undefinedAttribute*
Metalink is an extensible format. See Section 6 of this document for
a full description of how Metalink Documents can be extended.
3. Common Metalink Constructs
Many of Metalink's elements share a few common structures. This
section defines those structures and their requirements for
convenient reference by the appropriate element definitions.
When an element is identified as being a particular kind of
construct, it inherits the corresponding requirements from that
construct's definition in this section.
Note that there MUST NOT be any white space in a Date construct or in
any IRI. Some XML-emitting implementations erroneously insert white
space around values by default, and such implementations will emit
invalid Metalink Documents.
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Internet-Draft Metalink Download Description Format October 20093.1. Text Constructs
A Text construct contains human-readable text, usually in small
quantities. The content of Text constructs is Language-Sensitive.
metalinkTextConstruct =
metalinkCommonAttributes,
text
3.1.1. Text
Example metalink:description with text content:
...
<description>
A description of the example file for download.
</description>
...
The content of the Text construct MUST NOT contain child elements.
Such text is intended to be presented to humans in a readable
fashion. Thus, white space could be collapsed (including line
breaks) and text could be displayed using typographic techniques such
as justification and proportional fonts.
3.2. Date Constructs
A Date construct is an element whose content MUST conform to the
"date-time" production in [RFC3339]. In addition, an uppercase "T"
character MUST be used to separate date and time, and an uppercase
"Z" character MUST be present in the absence of a numeric time zone
offset.
metalinkDateConstruct =
metalinkCommonAttributes,
xsd:dateTime
Such date values happen to be compatible with the following
specifications: [ISO.8601.1988], [W3C.NOTE-datetime-19980827], and
[W3C.REC-xmlschema-2-20041028].
Example Date constructs:
<updated>2009-05-15T18:30:02Z</updated>
<updated>2009-05-15T18:30:02.25Z</updated>
<updated>2009-05-15T18:30:02+01:00</updated>
<updated>2009-05-15T18:30:02.25+01:00</updated>
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Date values SHOULD be as accurate as possible. For example, it would
be generally inappropriate for a publishing system to apply the same
timestamp to several Metalink Documents that were published during
the course of a single day.
4. Metalink Element Definitions4.1. Container Elements4.1.1. The "metalink:metalink" Element
The "metalink:metalink" element is the document (i.e., top-level)
element of a Metalink Document, acting as a container for metadata
and data associated with the listed files. It contains one or more
metalink:file child elements which consist of metadata elements.
metalinkMetalink =
element metalink:metalink {
metalinkCommonAttributes,
(metalinkDynamic?
& metalinkFile+
& metalinkGenerator?
& metalinkOrigin?
& metalinkPublished?
& metalinkUpdated?
& extensionElement*)
}
The following child elements are defined by this specification (note
that the presence of some of these elements is required):
o metalink:metalink elements MAY contain exactly one metalink:
dynamic element.
o metalink:metalink elements MUST contain one or more metalink:file
elements.
o metalink:metalink elements MAY contain exactly one metalink:
generator element.
o metalink:metalink elements SHOULD contain exactly one metalink:
origin element.
o metalink:metalink elements MAY contain exactly one metalink:
published element.
4.1.1.1. Providing Textual Content
Experience teaches that downloads providing textual content are in
general more useful than those that do not. Some applications (one
example is full-text indexers) require a minimum amount of text to
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function reliably and predictably. Metalink publishers should be
aware of these issues. It is advisable that each metalink:file
element contain a non-empty metalink:description element, a non-empty
metalink:identity element when that element is present, and a non-
empty metalink:version element, and a non-empty metalink:publisher
element. However, the absence of metalink:description is not an
error, and Metalink Processors MUST NOT fail to function correctly as
a consequence of such an absence.
4.1.2. The "metalink:file" Element
The "metalink:file" element represents an individual file, acting as
a container for metadata and data associated with the file.
All metalink:url elements SHOULD lead to identical files. That is,
each metalink:url element should be an alternative location for the
same file and each metalink:metaurl element should provide metadata
to retrieve the same file in another way, such as a peer to peer
network.
metalinkFile =
element metalink:file {
metalinkCommonAttributes,
attribute name { text },
(metalinkCopyright?
& metalinkDescription?
& metalinkHash*
& metalinkIdentity?
& metalinkLanguage?
& metalinkLicense?
& metalinkLogo?
& metalinkMetaURL*
& metalinkURL*
& metalinkOS*
& metalinkPieces*
& metalinkPublisher?
& metalinkSignature?
& metalinkSize?
& metalinkVersion?
& extensionElement*)
}
This specification assigns no significance to the order of metalink:
file elements or to the order of metalink:url or metalink:metaurl
elements. Significance is determined by the value of the "priority"
attribute of the metalink:url or metalink:metaurl elements.
The following child elements are defined by this specification (note
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that it requires the presence of some of these elements):
o metalink:file elements MAY contain exactly one metalink:copyright
element.
o metalink:file elements MAY contain exactly one metalink:
description element.
o metalink:file elements MAY contain exactly one metalink:identity
element.
o metalink:file elements MAY contain one or more metalink:hash
elements.
o metalink:file elements MAY contain exactly one metalink:language
element.
o metalink:file elements MAY contain exactly one metalink:license
element.
o metalink:file elements MAY contain exactly one metalink:logo
element.
o metalink:file elements MAY contain one or more metalink:os
element.
o metalink:file elements MUST contain at least one metalink:url
element or at least one metalink:metaurl element. Typically,
metalink:file elements contain more than one metalink:url element
to provide multiple download sources.
o metalink:file elements MAY contain one or more metalink:pieces
elements.
o metalink:file elements MAY contain exactly one metalink:publisher
element.
o metalink:file elements MAY contain one or more metalink:signature
elements.
o metalink:file elements SHOULD contain exactly one metalink:size
element.
o metalink:file elements MAY contain exactly one metalink:version
element.
4.1.2.1. The "name" Attribute
metalink:file elements MUST have a "name" attribute, which contains
the filename of the file to be downloaded.
Directory information can also be contained in a "path/file" format
only, as in:
<file name="debian-amd64/sarge/Contents-amd64.gz">
In this example, a subdirectory "debian-amd64/sarge/" will be created
and a file named "Contents-amd64.gz" will be created inside it. The
path MUST NOT contain any directory traversal directives or
information. The path MUST be relative. The path MUST NOT begin
with a "/", "./" or "../", contain "/../", or end with "/..".
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Internet-Draft Metalink Download Description Format October 20094.1.3. The "metalink:pieces" Element
The "metalink:pieces" element acts as a container for a list of
checksums of non-overlapping pieces of the file. The checksums MUST
be listed in the same order as the corresponding pieces appear in the
file, starting at the beginning of the file.
metalinkPieces =
element metalink:pieces {
attribute length { xsd:integer },
attribute type { text },
metalinkHash+
}
4.1.3.1. The "type" Attribute
metalink:pieces elements MUST have a "type" attribute.
The IANA registry named "Hash Function Textual Names" defines values
for hash types. If a Metalink Document contains hashes, it SHOULD
include "sha-1" which is SHA-1 as specified in [RFC3174], or a
stronger hash. It MAY also include other hashes.
4.1.3.2. The "length" Attribute
metalink:pieces elements MUST have a "length" attribute, which is an
integer that describes the length of the pieces of the file in
octets. The whole file is divided into non-overlapping pieces of
this length, starting from the beginning of the file. That is, every
piece should be the same size, apart from the last piece which is the
remainder. The last piece extends to the end of the file, and can
therefore be shorter than the other pieces.
4.2. Metadata Elements4.2.1. The "metalink:copyright" Element
The "metalink:copyright" element is a Text construct that conveys a
human-readable copyright for a file.
metalinkCopyright =
element metalink:copyright {
metalinkTextConstruct
}
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Internet-Draft Metalink Download Description Format October 20094.2.2. The "metalink:description" Element
The "metalink:description" element is a Text construct that conveys a
human-readable description for a file.
metalinkDescription =
element metalink:description {
metalinkTextConstruct
}
4.2.3. The "metalink:dynamic" Element
The "metalink:dynamic" element is a Text construct that describes
whether the IRI from "metalink:origin" in a Metalink will contain
dynamic updated information or if it is static and not likely to be
updated.
metalinkDynamic =
element metalink:dynamic {
"true" | "false"
}
4.2.4. The "metalink:generator" Element
The "metalink:generator" element's content identifies the generating
agent name and version, separated by a "/", used to generate a
Metalink Document, for debugging and other purposes.
metalinkGenerator =
element metalink:generator {
metalinkTextConstruct
}
The content of this element, when present, MUST be a string that is a
human-readable name and version, separated by a "/", for the
generating agent. For example, "MirrorBrain/2.9.2", where
"MirrorBrain" is the name and "2.9.2" is the version. Entities such
as "&amp;" and "&lt;" represent their corresponding characters ("&"
and "<" respectively), not markup.
4.2.5. The "metalink:hash" Element
The "metalink:hash" element is a Text construct that conveys a hash
for a file. All hashes are encoded in lowercase hexadecimal format.
Hashes are used to verify the integrity of a complete file or portion
of a file to determine if the file has been transferred without any
errors.
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metalinkHash =
element metalink:hash {
attribute type { text }?,
text
}
metalink:hash elements with a "type" attribute MUST contain a hash of
the complete file. Metalink Documents can contain one or multiples
hashes of a complete file. In this example, both SHA-1 and SHA-256
hashes are included.
...
<hash type="sha-1">a97fcf6ba9358f8a6f62beee4421863d3e52b080</hash>
<hash type="sha-256">fc87941af7fd7f03e53b34af393f4c14923d74
825f51116ff591336af4880227</hash>
...
Metalink Documents can also contain hashes for individual pieces of a
file. metalink:hash elements that are inside a metalink:pieces
container element have a hash for that specific piece or chunk of the
file, and are of the same hash type as the metalink:pieces element
they are contained in. metalink:hash elements without a "type"
attribute MUST contain a hash for that specific piece or chunk of the
file and MUST be listed in the same order as the corresponding pieces
appear in the file, starting at the beginning of the file. The size
of the piece is equal to the value of the "length" attribute of the
metalink:pieces element. The whole file is divided into non-
overlapping pieces of this length, starting from the beginning of the
file. That is, every piece should be the same size, apart from the
last piece which is the remainder. The last piece extends to the end
of the file, and can therefore be shorter than the other pieces.
...
<hash type="sha-1">a97fcf6ba9358f8a6f62beee4421863d3e52b080</hash>
<hash type="sha-256">fc87941af7fd7f03e53b34af393f4c14923d74
825f51116ff591336af4880227</hash>
<pieces length="1048576" type="sha-1">
<hash>d96b9a4b92a899c2099b7b31bddb5ca423bb9b30</hash>
<hash>10d68f4b1119014c123da2a0a6baf5c8a6d5ba1e</hash>
<hash>3e84219096435c34e092b17b70a011771c52d87a</hash>
<hash>67183e4c3ab892d3ebe8326b7d79eb62d077f487</hash>
</pieces>
...
4.2.5.1. The "type" Attribute
metalink:hash elements MUST have a "type" attribute, if and only if
it contains a hash of the complete file. The IANA registry named
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"Hash Function Textual Names" defines values for hash types. If a
Metalink Document contains hashes, it SHOULD include "sha-1" which is
SHA-1 as specified in [RFC3174], or a stronger hash. It MAY also
include additional hash types.
4.2.6. The "metalink:identity" Element
The "metalink:identity" element is a Text construct that conveys a
human-readable identity for a file. The identity of OpenOffice.org
3.0 would be "OpenOffice.org".
metalinkIdentity =
element metalink:identity {
metalinkTextConstruct
}
4.2.7. The "metalink:language" Element
The "metalink:language" element is a Text construct that conveys a
code for the language of a file, per [RFC4646].
metalinkLanguage =
element metalink:language {
metalinkTextConstruct
}
4.2.8. The "metalink:license" Element
The "metalink:license" element is a Text construct that conveys a
human-readable license name for a file.
metalinkLicense =
element metalink:license {
metalinkCommonAttributes,
attribute url { metalinkUri }?,
attribute name { text }?
}
The metalink:license element MAY have a "url" attribute whose value
MUST be an IRI reference [RFC3987]. When dereferenced, the resulting
URI (mapped from an IRI, if necessary) SHOULD produce a
representation that is relevant to that agent.
The metalink:license element MAY have a "name" attribute that
indicates the name of the license.
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Internet-Draft Metalink Download Description Format October 20094.2.9. The "metalink:logo" Element
The "metalink:logo" element's content is an IRI reference [RFC3987]
that identifies an image that provides visual identification for a
file.
metalinkLogo =
element metalink:logo {
metalinkCommonAttributes,
(metalinkUri)
}
The image SHOULD have an aspect ratio of one (horizontal) to one
(vertical) and SHOULD be suitable for presentation at a small size.
4.2.10. The "metalink:metaurl" Element
The "metalink:metaurl" element contains the IRI of metadata about a
resource to download. For example, this could be the IRI of a
BitTorrent .torrent file or a Metalink Document. Note that the
information in the metalink:hash element does not apply to these
files, but to the files that are described by them.
metalinkMetaURL =
element metalink:metaurl {
metalinkCommonAttributes,
attribute priority { xsd:integer }?,
attribute type { text },
attribute name { text }?,
metalinkUri
}
4.2.10.1. The "priority" Attribute
metalink:metaurl elements MAY have a priority attribute. Lower
values indicate a higher priority. Multiple metalink:metaurl
elements can have the same priority, i.e. three BitTorrent .torrent
files could have priority="1". See also the "priority" attribute of
the metalink:url element.
4.2.10.2. The "type" Attribute
metalink:metaurl elements MUST have a "type" attribute that indicates
the MIME type of the metadata available at the IRI. In the case of
BitTorrent as specified in [BITTORRENT], the value "torrent" is
required. Types without "/" are reserved. Currently, "torrent" is
the only reserved value.
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Internet-Draft Metalink Download Description Format October 20094.2.10.3. The "name" Attribute
metalink:metaurl elements MAY have a "name" attribute that indicates
a specific file in a BitTorrent .torrent file or a Metalink Document
that describes multiple files.
Directory information can also be contained in a "path/file" format
only, as in:
<metaurl type="torrent" name="debian-amd64/sarge/Contents-amd64.gz">
In this example, a file named "Contents-amd64.gz" is indicated, in a
"debian-amd64/sarge/" subdirectory. The path MUST NOT contain any
directory traversal directives or information. The path MUST be
relative. The path MUST NOT begin with a "/", "./" or "../", contain
"/../", or end with "/..".
4.2.11. The "metalink:origin" Element
The "metalink:origin" element is an IRI where the Metalink Document
was originally published. If metalink:dynamic is "true", then
updated versions of the Metalink can be found at this IRI.
metalinkOrigin =
element metalink:origin {
metalinkCommonAttributes,
(metalinkUri)
}
4.2.12. The "metalink:os" Element
The "metalink:os" element is a Text construct that conveys a human-
readable Operating System for a file. The IANA registry named
"Operating System Names" defines values for OS types.
metalinkOS =
element metalink:os {
metalinkTextConstruct
}
4.2.13. The "metalink:published" Element
The "metalink:published" element is a Date construct indicating an
instant in time associated with an event early in the life cycle of
the entry.
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metalinkPublished =
element metalink:published {
metalinkDateConstruct
}
Typically, metalink:published will be associated with the initial
creation or first availability of the resource.
4.2.14. The "metalink:publisher" Element
The "metalink:publisher" element indicates a group or other entity
which has published the file described in the Metalink Document.
metalinkPublisher =
element metalink:publisher {
metalinkCommonAttributes,
attribute url { metalinkUri }?,
attribute name { text }?
}
The metalink:publisher element MAY have a "url" attribute whose value
MUST be an IRI reference [RFC3987]. When dereferenced, the resulting
URI (mapped from an IRI, if necessary) SHOULD produce a
representation that is relevant to that agent.
The metalink:publisher element MAY have a "name" attribute that
indicates the name of the publisher.
4.2.15. The "metalink:signature" Element
The "metalink:signature" element is a Text construct that conveys a
digital signature for a file described in a Metalink Document.
Digital signatures verify that a file is from the entity that has
signed it.
metalinkSignature =
element metalink:signature {
attribute type { "pgp" },
metalinkTextConstruct
}
4.2.15.1. The "type" Attribute
metalink:signature elements MUST have a "type" attribute. The
initial value of "type" is the string that is non-empty and matches
"pgp". It may be useful to extend Metalink documents with new types
of digital signatures, so unknown types are allowed.
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Internet-Draft Metalink Download Description Format October 20094.2.16. The "metalink:size" Element
The "metalink:size" element indicates the length of the linked
content in octets; it is a hint about the content length of the
representation returned when the IRI is mapped to a URI and
dereferenced.
metalinkSize =
element metalink:size {
metalinkTextConstruct
}
4.2.17. The "metalink:updated" Element
The "metalink:updated" element is a Date construct indicating the
most recent instant in time when a Metalink was modified in a way the
publisher considers significant. Therefore, not all modifications
necessarily result in a changed metalink:updated value.
metalinkUpdated =
element metalink:updated {
metalinkDateConstruct
}
Publishers MAY change the value of this element over time.
4.2.18. The "metalink:url" Element
The "metalink:url" element contains the IRI of a file. Most Metalink
Documents will contain multiple metalink:url elements, and each one
SHOULD be a valid alternative to download the same file.
metalinkURL =
element metalink:url {
metalinkCommonAttributes,
attribute location { xsd:string {
minLength = "2" maxLength="2"}
}?,
attribute priority { xsd:integer }?,
metalinkUri
}
4.2.18.1. The "priority" Attribute
metalink:url elements MAY have a priority attribute. Lower values
indicate a higher priority. Multiple metalink:url elements can have
the same priority, i.e. ten mirrors could have priority="1".
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Internet-Draft Metalink Download Description Format October 20094.2.18.2. The "location" Attribute
metalink:url elements MAY have a "location" attribute, which is a
[ISO3166-1] alpha-2 two letter country code for the geographical
location of the physical server an IRI is used to access.
4.2.19. The "metalink:version" Element
The "metalink:version" element is a Text construct that conveys a
human-readable version for a file. The version of OpenOffice.org 3.0
would be "3.0".
metalinkVersion =
element metalink:version {
metalinkTextConstruct
}
5. Securing Metalink Documents
Because Metalink is an XML-based format, existing XML security
mechanisms can be used to secure its content.
Producers of Metalink Documents may have sound reasons for signing
otherwise-unprotected content. For example, a merchant might
digitally sign a Metalink that lists a file download to verify its
origin. Other merchants may wish to sign and encrypt Metalink
Documents that list digital songs that have been purchased. Of
course, many other examples are conceivable as well.
The algorithm requirements in this section pertain to the Metalink
Processor. They require that a recipient, at a minimum, be able to
handle messages that use the specified cryptographic algorithms.
These requirements do not limit the algorithms that the sender can
choose.
Metalink Processors that verify signed Metalink Documents MUST at
least support XML-Signature and Syntax Processing [REC-xmldsig-core].
6. Extending Metalink6.1. Extensions from Non-Metalink Vocabularies
This specification describes Metalink's XML markup vocabulary.
Markup from other vocabularies ("foreign markup") can be used in an
Metalink Document.
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Internet-Draft Metalink Download Description Format October 20096.2. Extensions to the Metalink Vocabulary
The Metalink namespace is reserved for future forward-compatible
revisions of Metalink. Future versions of this specification could
add new elements and attributes to the Metalink markup vocabulary.
Software written to conform to this version of the specification will
not be able to process such markup correctly and, in fact, will not
be able to distinguish it from markup error. For the purposes of
this discussion, unrecognized markup from the Metalink vocabulary
will be considered "foreign markup".
6.3. Processing Foreign Markup
Metalink Processors that encounter foreign markup in a location that
is legal according to this specification MUST NOT stop processing or
signal an error. It might be the case that the Metalink Processor is
able to process the foreign markup correctly and does so. Otherwise,
such markup is termed "unknown foreign markup".
When unknown foreign markup is encountered as a child of metalink:
file, metalink:metalink, Metalink Processors MAY bypass the markup
and any textual content and MUST NOT change their behavior as a
result of the markup's presence.
When unknown foreign markup is encountered in a Text Construct,
software SHOULD ignore the markup and process any text content of
foreign elements as though the surrounding markup were not present.
6.4. Extension Elements
Metalink allows foreign markup anywhere in an Metalink document,
except where it is explicitly forbidden. Child elements of metalink:
file and metalink:metalink are considered Metadata elements and are
described below. Child elements of Person constructs are considered
to apply to the construct. The role of other foreign markup is
undefined by this specification.
6.4.1. Simple Extension Elements
A Simple Extension element MUST NOT have any attributes or child
elements. The element MAY contain character data or be empty.
Simple Extension elements are not Language-Sensitive.
simpleExtensionElement =
element * - metalink:* {
text
}
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Internet-Draft Metalink Download Description Format October 2009
The element can be interpreted as a simple property (or name/value
pair) of the parent element that encloses it. The pair consisting of
the namespace-URI of the element and the local name of the element
can be interpreted as the name of the property. The character data
content of the element can be interpreted as the value of the
property. If the element is empty, then the property value can be
interpreted as an empty string.
6.4.2. Structured Extension Elements
The root element of a Structured Extension element MUST have at least
one attribute or child element. It MAY have attributes, it MAY
contain well-formed XML content (including character data), or it MAY
be empty. Structured Extension elements are Language-Sensitive.
structuredExtensionElement =
element * - metalink:* {
(attribute * { text }+,
(text|anyElement)*)
| (attribute * { text }*,
(text?, anyElement+, (text|anyElement)*))
}
The structure of a Structured Extension element, including the order
of its child elements, could be significant.
This specification does not provide an interpretation of a Structured
Extension element. The syntax of the XML contained in the element
(and an interpretation of how the element relates to its containing
element) is defined by the specification of the Metalink extension.
7. IANA Considerations7.1. XML Namespace Registration
This document makes use of the XML registry specified in [RFC3688].
Accordingly, IANA has made the following registration:
Registration request for the Metalink namespace:
URI: urn:ietf:params:xml:ns:metalink
Registrant Contact: See the "Author's Address" section of this
document.
XML: None. Namespace URIs do not represent an XML specification.
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Internet-Draft Metalink Download Description Format October 20097.2. application/metalink4+xml MIME type
A Metalink Document, when serialized as XML 1.0, can be identified
with the following media type:
MIME media type name: application
MIME subtype name: metalink4+xml
Mandatory parameters: None.
Optional parameters:
"charset": This parameter has semantics identical to the charset
parameter of the "application/xml" media type as specified in
[RFC3023].
Encoding considerations: Identical to those of "application/xml" as
described in [RFC3023], Section 3.2.
Security considerations: As defined in this specification.
In addition, as this media type uses the "+xml" convention, it
shares the same security considerations as described in [RFC3023],
Section 10.
Interoperability considerations: There are no known interoperability
issues.
Published specification: This specification.
Applications that use this media type: No known applications
currently use this media type.
Additional information:
Magic number(s): As specified for "application/xml" in [RFC3023],
Section 3.2.
File extension: .meta4
Fragment identifiers: As specified for "application/xml" in
[RFC3023], Section 5.
Base URI: As specified in [RFC3023], Section 6.
Macintosh File Type code: TEXT
Person and email address to contact for further information: Anthony
Bryan <anthonybryan@gmail.com>
Intended usage: COMMON
Author/Change controller: IESG
8. Security Considerations
Publishers are encouraged to offer Metalink documents via
authenticated HTTP under TLS as specified in [RFC2818]. Publishers
are also encouraged to include digital signatures of the files within
the Metalink Documents if they are available.
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Internet-Draft Metalink Download Description Format October 20098.1. URIs and IRIs
Metalink Processors handle URIs and IRIs. See Section 7 of [RFC3986]
and Section 8 of [RFC3987] for security considerations related to
their handling and use.
8.2. Spoofing
There is potential for spoofing attacks where the attacker publishes
Metalink Documents with false information. Malicious publishers
might create Metalink Documents containing inaccurate information
anywhere in the document. Unaware downloaders could be deceived into
downloading a malicious or worthless file. Malicious publishers
could attempt a distributed denial of service attack by inserting
unrelated IRIs into Metalink Documents.
8.3. Cryptographic Hashes
Currently, some of the hash types defined in the IANA registry named
"Hash Function Textual Names" are considered insecure. These include
the whole Message Digest family of algorithms which are not suitable
for cryptographically strong verification. Malicious people could
provide files that appear to be identical to another file because of
a collision, i.e. the weak cryptographic hashes of the intended file
and a substituted malicious file could match.
If a Metalink Document contains hashes, it SHOULD include "sha-1"
which is SHA-1, as specified in [RFC3174], or stronger. It MAY also
include other hashes.
8.4. Signing
Metalink Documents SHOULD be signed using [REC-xmldsig-core] and are
subject to the security considerations implied by its use. This
addresses the issue of spoofing.
Digital signatures provide authentication, message integrity, and
non-repudiation with proof of origin.
9. References9.1. Normative References
[BITTORRENT]
Cohen, B., "The BitTorrent Protocol Specification",
BITTORRENT 11031, February 2008,
<http://www.bittorrent.org/beps/bep_0003.html>.
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